Apollo 1 disaster in 1967

Three astronauts, Lt. Col. Virgil I. Grissom, Lt. Col. Edward H. White, the astronaut who had performed the first United States extravehicular activity during the Gemini program; and Roger B. Chaffee, an astronaut preparing for his first space flight, died during a test when a fire swept through the capsule. They died of cardiac arrest brought on by high levels of carbon monoxide.

The review board charged with investigating the accident identified five major factors which combined to cause the fire and the astronauts' deaths:

• An ignition source most probably related to exposed electrical wiring and leak-prone plumbing
• Pure oxygen atmosphere at high pressure
• Flammable materials in the cabin
• A hatch cover which could not be quickly removed at high pressure
• Inadequate emergency preparedness

The Apollo program was grounded for a while until significant  changes to the design of the capsule could be conducted.